Process for producing anisotropic graphite-metal composites

ABSTRACT

AN ANISOTROPIC GRAPHITE-METAL COMPOSITE OF VERMICULAR EXPANDED GRAPHITE CONTAINING FROM 5 TO 75 WEIGHT PERCENT, BASED ON THE TOTAL WEIGHT OF THE COMPOSITE, OF A METAL POWDER HAVING A PARTICLE SIZE LESS THAN 100 MESH AND COMPRESSED TO A DENSITY OF 10 TO 120 POUNDS PER CUBIC FOOT. SUCH COMPOSITES ARE PREPARED BY ADMIXING VERMICULAR EXPANDED GRAPHITE WITH FROM 5 TO 75 WEIGHT PERCNET OF SILVER, COPPER, NICKEL, ZINC, LEAD, SILICON, IRON OR BORON HAVING A PARTICLE SIZE OF LESS THAN 100 MECH AND COMPRESSING THE MIXTURE TO A DENSITY OF FROM 10 TO 120 POUNDS PER CUBIC FOOT. TO ACHIEVE ADDITIONAL STRENGTH THE COMPRESSED COMPOSITE IS HEATED TO SINTER THE METAL POWDER.

3,666,455 Patented May 30, 1972 Int. Cl. B22f 7/00 U.S. Cl. 75-201Claims ABSTRACT OF THE DISCLOSURE An anisotropic graphite-metalcomposite of vermicular expanded graphite containing from 5 to 75 weightpercent, based on the total weight of the composite, of a metal powderhaving a particle size less than 100 mesh and compressed to a density of10 to 120 pounds per cubic foot. Such composites are prepared byadmixing vermicular expanded graphite with from 5 to 75 weight percentof silver, copper, nickel, zinc, lead, silicon, iron or boron having aparticle size of less than 100 mesh and compressing the mixture to adensity of from 10 to 120 pounds per cubic foot. To achieve additionalstrength the compressed composite is heated to sinter the metal powder.

This application is a divisional of application Ser. No. 441,905, filedMar. 22, 1965, now Pat. No. 3,492,197, granted Jan. 27, 1970.

This invention relates to a method of preparing novel forms ofcompressed graphite and more particularly is concerned with a processfor compressing expanded graphite into various shapes and forms whereinthe resulting graphitic structure has extremely desirable properties, oronto various substrates or supports, and to the products preparedthereby.

A principal object of the instant invention is to provide a method forproducing novel forms of compressed graphite.

An additional object is to provide a method for producing novelcompressed forms and shapes of graphite which have unexpected desirableproperties including, for example, high electrical and thermalconductivity, high or low anisotropic ratios, low liquid and gaspermeabilities, resistance to high temperature oxidation, and excellentmechanical properties.

These and other objects and advantages of the instant invention willbecome apparent from reading the detailed description thereof set forthhereinafter.

In accordance with the instant invention, novel forms of graphite areprepared by providing a supply of an expanded particulate graphite andcompressing said expanded graphite at predetermined pressures anddirections into predetermined forms of cohered graphite.

Expanded graphite used in the present invention is prepared fromparticulate naturally occurring crystalline flake graphite orcrystalline lump graphite, flake graphite being preferred. Thecystalline graphite is given a particular acid treatment and theso-treated flake is heated at certain operable temperatures therebyexpanding into the low density vermicular feed stock suitable for use inthe present invention. The particle size of graphite to be used is notcritical although ordinarily particles of from about 10 to about 325mesh US. Standard Sieve are used. Larger vflakes (about 10 to 50 mesh)are generally preferred, however, because as a general rule strongercompacted articles are produced from the expanded graphite prepared fromthese flakes. Further, larger flakes give a low density expandedgraphite than smaller flakes.

More particularly, the present invention comprises providing a supply ofan expanded vermicular graphite having an apparent bulk density as lowasabout 0.1 pound per cubic foot (lb./ft. and no more than about 10lb./ft. and preferably within the range of from about 0.2 to about 2lb./ft. compresing the expanded vermicular graphite at pressures fromabout 5 to about 50,000 pounds per square inch (p.s.i.) or more, inpredetermined directions in a mold or other'forming apparatus. Theactual pressure applied and the manner of pressure application (that is,uniaxially, biaxially, or triaxially), is dependent onthe type of formedproduct desired or required. As the applied pressure is increased on theexpanded vermicular graphite the bulk density of the resulting compactincreases correspondingly. To illw trate, a compressed fabricationhaving a bulk density of about 10 lb./ft. results at an applied pressureof about 20 p.s.i.; the product exhibits a bulk density of from about toabout lb./ft. at applied pressures in the order of 4000 to 25,000 p.s.i.or higher. Other properties of the resulting compacted article also canbe varied depending on the degree of compaction,- for example, increasedelectrical and thermal conductivities result with increasing formationpressures in theplane perpendicular to the compression vector, andimproved mechanical properties (for example tensile and compressivestrengths) are realized at higher pressures. An unexpected but valuableadvantage of the present invention is the production of fabrications atincreasing pressures which exhibit thermal and electrical anisotropy.The electrical anisotropy (that is the ratio of electrical resistancesparallel to and normal to the applied compression vector(s)) of thecompacts increase from a value of unity at about 10 p.s.i. appliedpressure up to about 250 or more when applied pressures reach 10,000p.s.i. or more.

The present invention is particularly suitable for preparing compressedvermicular graphite structures exhibiting excellent anisotropicproperties. Such a graphite compact is prepared in accordance with theinstant invention by uniaxially compressing the expanded vermiculargraphite. Such products exhibit an electrical anisotropic ratio as highas 300:1 and a thermal anisotropic ratio as high as 300:1 compared to amaximum of only about 4:1 and 4:1, respectively, for a conventional formof synthetic (Acheson-Process type) polycrystalline graphite atcomparable densities. In addition, the compressed vermicular graphiteproduct of the instant invention can yield higher densities than theconventional synthetic polycrystalline graphite. These densities may beup to and approaching the theoretical density of graphite. Otheradvantages include a lower gas permeability to helium, a product whichis more resistant to high temperature oxidation, has a higher thermalconductivity (as much as two or more times higher than copper below 60C. and about six or more times higher than synthetic (Acheson- Processtype) polycrystalline graphite), and has a higher electricalconductivity (two or three times higher than (Acheson-Process type)synthetic graphite.

A further advantage of the method of the instant invention incorporatinguniaxial compression on the vermicular expanded graphite is that it canproduce a highly flexible, impermeable graphite foil or sheet having athickness as low as about 0.001 inch. Using expanded graphite having adensity of from about 0.2 to about 2.0 lb./ft. pressures of from about100 to about 50,000 p.s.i. should be applied hereto in obtaining thesefoils. In addition to being highly flexible, the foil unexpectedly has avery high thermal and electrical conductivity in the plane of the foil(normal to the compression vector) but behaves as a thermal andelectrical insulator across the thickness of the foil.

Compressed expanded vermicular graphitic structures havingnear-isotropic properties can also be made in accordance with theinstant method. Such structure with reduced electrical and thermalanisotropic properties can be prepared by biaxially compressing expandedvermicular graphite, (i.e. in two mutually perpendicular axessequentially or simultaneously). The vector of high thermal andelectrical conductivity is in a direction perpendicular to the biaxialcompression vectors. The anisotropic ratio (electrical or thermal) ofthese biaxially compressed graphite structures can be controlled by thepressure applied in each of the two directions during the biaxialcompression. For example, it slight pressure is applied along an axis aand great pressure applied along an axis b (which is perpendicular toaxis a), axis (which is perpendicular to axes a and b) would be thevector of high thermal and electrical conductivity. The minimumanisotrophy ratio would occur between the c and a axes.

The electrical and thermal conductivity values in the high conductivitydirection of the biaxially compressed graphite compacts areapproximately equal to the values in the high conductivity direction inthe uniaxially compressed vermicular graphite (at equal densities), butthe low conductivity vector values of biaxially compressed graphite ismany times more conductive than the values of the low conductivityvector of uniaxially compressed vermicular graphite. Therefore, thebiaxially compressed vermicular graphite exhibits the improvedcharacteristics of uniaxially compressed vermicular graphite without thehigh anisotropy in applications where high electrical or thermalanisotropy would be objectionable. In addition, the uniaxiallycompressed material has lower tensile strength in the direction parallelto the compression vector than in the direction normal to thecompression vector. However, the biaxially compressed graphite has aseveral fold increase in tensile strength in its weak direction.

In accordance with the instant method, a volume of expanded vermiculargraphite can be placed in a vessel so that a chosen axis or directionwill lie in such a manner that when radial compression forces areapplied (such as by using pressurized fluids), the compression forcesact directly on all surfaces except those at the ends of the chosenaxis; that is, the compression force is isostatic except at the ends ofthe so chosen axis. For example,

a cylinder of expanded vermicular graphite is compressed,

along the radius, but the compression forces are prevented from actingon the axis of the cylinder by keeping the ends of the solid cylinder ina fixed position. This yields a compact having high thermal andelectrical conductivity parallel to the axis of the solid cylinder.

The properties of a compressed (e.g. at about 100 p.s.i. or greater)graphite structure can be made more nearly isotropic by triaxiallycompressing the expanded vermicular graphite (along mutuallyperpendicular axes), seqentially or simultaneously. Thus, for example,triaxially compressed dense graphite compacts can be made in accordancewith the present method which have electrical conductivity valuesessentially equal in all three mutually perpendicular axes (i.e.perfectly isotropic). Biaxially and uniaxially compressed dense graphitecompacts have unequal conductivity values between at least two of thethree mutually perpendicular axes. It is to be understood thattriaxially compressed vermicular graphite does not show the highelectrical and thermal conductivities in a preferred direction and itmay not show anisotropic properties as does uniaxially and biaxiallycompressed vermicular graphite (at the same density) but rather may showproperties close to that exhibited by commercial Acheson-Process typepolycrystalline graphite. However, advantageously, the triaxiallycompressed vermicular graphite can show higher density, lowerpermeability and less brittleness than synthetic Acheson-Process typepolycrystalline graphite.

It is to be understood that the amount of anisotropy,

4 the density, and the permeability of the dense compressed compacts isdependent on the amount of the compression forces employed, and onwhether or not equal compression forces are used on the mutuallyperpendicular axes.

As With the uniaxially compressed material, biaxially and triaxiallycompressed vermicular graphite structures of various desired densities,up to about the theoretical limiting density of graphite (2.25 gms./cc.)readily are prepared.

Additionally, high density bonded-expanded vermicular graphite coheredstructures exhibiting marked thermal and electrical anisotropicproperties and mechanical properties can be prepared in accordance withthe instant invention. In preparing such structures, the expandedvermicular graphite ordinarily is blended with an inorganic or organicbonding agent, ordinarily in the form of a fine powder, in an amount offrom about 2 to about 55 Weight percent and preferably from about 5 toabout 45 Weight percent bonding agent based on the total weight of thegraphite-binder mixture. This mass is then compressed at pressuresgreater than 1000 p.s.i. and usually from about 10,000 to about 25,000psi to the desired density in the appropriately shaped mold or form. Theso-formed composite is then treated to activate the bonding agent andpromote adhesion within the compact.

The bonding agents are also useful in preparing low density articles inwhich compaction forces of from about 5 to 1000 psi. are employed priorto activating the bonding agent.

Solid bonding agents ordinarily should be in the form of a fine powderand have a particle size of less than mesh and preferably from about 200to about 325 mesh.

Bonding agents which ordinarily are used in the instant invention arethermoplastic or thermodegradable materials and include (1) any solidorganic polymer (2) other organic compounds which, upon pyrolysis, yielda cementing char (3) inorganic glass-like bonding agents and the like.

Examples of organic polymers suitable for use herein include but are notlimited to polyethylene, acrylic and methacrylic polymers, polystyrene,epoxides, polyvinyl chloride, polyesters, polycarbonates, phenolformaldehydes, nylon, polytetrafluoroethylene, polyvinylidenefiuoride,copolymers of the same, and the like. These bonding agents can be usedalong with any required catalyst or crosslinker.

Examples of such other organic char yielding substances suitable for useherein include coal tar pitches, natural asphalts, phenol-formaldehyde,urea-formaldehyde, polyvinylidene chloride, and copolymers containingpolyvinylidene chloride, polymers of furfuryl alcohol,polyacrylonitrile, sugars, saccharides and the like.

Examples of inorganic glass bonding agents suitable for use herein arevitreous materials which include, glassforming oxides such as boricoxide, silica, phosphorous pentoxide, germanium oxides, vanadiumpentoxide, and the like or other inorganic salts that can be obtained asglasses such as beryllium fluoride, and certain sulfates, chlorides andcarbonates. Especially useful in this invention are those glass-formerswhich will wet the graph rte, such as B 0 P 0 or V 0 Commerciallyavailable glasses also are suitable as bonding agents. Typical examplesof such glasses are compositions containing as an ingredient variousproportions of two or more of the following oxides: silica, aluminumoxide, sodium oxide, potassium oxide, magnesium oxide, cuprous oxide,barium oxide, lead oxide, or boric oxide.

Glass-forming oxides are defined as those oxides which are indispensableto the formation of oxide glasses. Those skilled in the art ofglass-making will readily recognize that the above named oxides aregenerally employed in combination with other materials to obtain glass.

It has been found that certain inorganic compounds can be blended withexpanded graphite prior to the compaction of the graphite into thedesired articles, the in low density vermicular graphite, placing ametal mesh on this layer and applying an additional layer of vermiculargraphite to the upper surface of the metal mesh. The system is thencompressed under at least 500 p.s.i. and preferably above 10,000 p.s.i.to produce a cohered mesh-reinforced graphite sheet. Any flexible metalmesh may be employed but copper, bronze, steel, aluminum and nickel meshare generally desirable. The form of the mesh is not critical and may bewoven or may simply be a perforate foil. Metal mesh-graphite compositessuch as this are useful as high temperature gaskets, packing materials,thermal radiation shields, flexible conductors and the like.

A convenient method of applying a thin essentially continuous coating ofgraphite onto a substrate is to first coat the substrate with a tackyadhesive or bonding agent, then rub or press the expanded graphite ontothe adhesive layer.

The products produced by the instant invention have varied usesdepending on the shape, or form, and properties they possess. Whenvermicular expanded graphite is compressed in a suitable mold, shapessuch as plates, rectangular solids, cones, rods, spheres, thin foils,hollow hemispheres and other more complicated configurations are formed.These solids find utility as thermal conductors or insulators, improvedelectrical conductors and in directional control of heat or electriccurrent flow.

Articles made according to this invention find particularly goodapplication as sealing agents, such as sealing rings, valve packing, andgaskets. Such articles may or may not be prepared in such a manner as toexhibit an anisotropic conductance property. Also such articles arecapable of undergoing some deformation under high compression forceswithout shattering or breaking, whereas articles prepared for the samepurpose by using the Acheson-Process type crystalline graphite orpyrolytic graphite are quite brittle and will shatter or break easilyunder commonly encountered deformation forces when used as gasketing orpacking material.

The highly anisotropic flexible foil produced by uniaxial compressioncan be used as a flexible electrical resistance element or an electricalstrip heater as gasketing material or chemically inert, low liquidorgas-permeability sheet stock for lining vessels, pipes, and equipmentsurfaces by bonding said foil with conventional adhesives to the surfaceto be protected. In the same manner, the anisotropic characteristics ofrods, bars, tubes and the like may be used to produce heating elementsfrom the compressed graphite so long as the current path is parallel tothe compression vector of the element.

Furthermore, a strip or sheet of compressed vermicular graphite can beemployed as a heating element for a rug or carpet. The expandedvermicular graphite prepared as described hereinbefore, can bepre-formed into a continuous sheet before being placed against therug-backing or can be compressed in situ against the backing after thegraphite is introduced. An outer backing should then be applied to thecompressed graphite layer. Electrical contact is established from thecompressed vermicular graphite layer of the rug backing to a powersource.

One manner of obtaining uniform electrical flow through the layer ofcompressed graphite thereby heating the rug or carpet is by embedding ametal screen wire of good conductivity in the graphite layer near eachend of the graphite sheet, extending laterally from side to side for thepurpose of distributing the electric current uniformly through thegraphite. Each screen is wired to a power source.

The backing material can be any of the flexible nonconductive backingscommon to the rug-making industry. Further, the backing material can bereplaced by or supplemented with a foam or cushion insulating materialto retard the loss of heat from the rug into the floor beneath it.

The compressed graphite should be intimately adhered to the backingeither by applying the graphite to the backing while the backing hasadhesive properties or by employing an adhesive to hold the backing andgraphite together. The outer backing itself should be adhered to thegraphite for example by water-based or solvent-based adhesives. It ispreferred that the outer backing be adhered to the inner backing atrandom points through small holes in the graphite layer. Such randomadhesion between the two backing layers can be obtained, for example, byemploying a water-based backing material (such as a latex) or asolvent-based material (such as a solution of backing material in asolvent). The liquid or fluid backing systems can be applied while inthe liquid state, thereby obtaining sufficient impregnation of theperforated graphite material so that random adhesion between the backinglayers is obtained.

Generally, expanded graphite, in an amount of about 0.5 to about 5 gramsper square foot of rug backing should be used. The electrical conductionof the graphite strip or backing increases as its density and/orcross-section increases. Thus the thickness of the graphite layer to beused can be predetermined to obtain the desired resistance so that thedesired amount of heat will be emitted when an electric current ispassed through the graphite layer. For example, in order to maintain atemperature of from about 75 to about 150 F. within the rug or carpetfor a 110-115 volt A.C. source (a wattage of about 5 to about 60 wattsper square foot of rug area would be necessary) the thickness of thegraphite layer should be about 0.005 to about 0.05 inch, if ambienttemperature is about 70 F.

The highly anisotropic bonded structures and laminates find particularuse in the manufacture of atmospheric re-entry nose cones for missiles,in the design of nozzles for solid fuel rocket motors, in thefabrication of improved electrical commutation brushes and in theconstruction of compact variable resistors.

The highly anisotropic supported graphite surfaces find utility as flat,graphite electrical conductors for use in corrosive environments, inelectrical heating devices such as blankets, heating trays, domestic orindustrial space heaters, process strip heaters and the like.

Compacts of expanded graphite and metal prepared by mixing the graphitewith fine metal powder and subsequently compressing in one or morevectors produces highly conductive compacts which also show a highdegree of anisotropy. Mixtures of graphite with from 5 to 75 percent byweight of a fine (preferably at least 100 mesh) metal powder producecompacts having exceptionally high anisotropic ratios when compared toother metal compacts. Such powders as copper, silver, zinc, nickel,lead, silicon, iron and boron may be used. Alternately, easily reduciblesalts of metals may be used. The compacts thus produced have goodphysical strength but improved physical strength may be produced byheating the compact to or near the melting point of the metal componentto fuse or sinter it. Also oganic or inorganic binders may be employedif desired. Such compacts find utility as electrical commutator brushes,polarizers for electromagnetic radiation and the like.

In preparing the expanded graphite, for use in the present invention, aparticulate natural crystalline graphite is contacted at about roomtemperature with (l) a mixture of from about 8 to about 98 weightpercent concentrated sulfuric acid (at least about 90 weight percent Hand from about 92 to about 2 weight percent concentrated nitric acid (atleast about 60 weight percent HNO or (2) fuming nitric acid, or (3)fuming sulfuric acid, or (4) concentrated sulfuric acid (at least aboutweight percent H 504) or concentrated nitric acid (at least about 60weight percent HNO plus at least about 2 weight percent of a solidinorganic oxidizer such as, for example, manganese dioxide, potassiumpermanganate, chromium trioxide, potassium chlorate and the like. Theresulting mix components usually are emorganic compounds serving toincrease the resistance of the graphite to high temperature oxidation.The com pounds found to serve this purpose include B P 0 Ca (PO AlP0 Zn(PO and Na B O The compounds useful as oxidation-resistant additives mayor may not be potential constituents of glass-forming formulations. Thematerials should be in a particulate form which will pass through aIOU-mesh screen and preferably a 325-mesh screen. The effectiveconcentrations of said oxidation-resistant additives generally are inthe range of about 0.5 weight percent to about weight percent of thegraphite charge. At concentrations below about 0.5 weight percent theoxidation rate of the graphite article approaches that of the graphitearticle alone and at concentrations above about 10 weight percent thereis no significant decrease in oxidation rate of the graphite and theinorganic additive begins to affect the physical properties of thegraphite.

Liquid polymers usually are not employed as the bonding agent sincethese can prevent formation of highly densified composites, but may beadvantageously employed wherein a low density compact is desired and theliquid polymer is hardenable, curable, or capable of being set-up.However, a solid polymer or other bonding agents can be dissolved in asolvent and then sprayed on the vermicular expanded graphite prior tocompaction. When this technique is used to make dense compacts thesolvent is removed before compaction is attempted. Solvents suitable foruse in dissolving solid polymers include, for example xylene, kerosene,CCl acetone and the like.

The uniaxially, biaxially or triaxially compressed vermicular graphitecompacts have properties vastly superior to Acheson-Process typepolycrystalline graphite. However, desirable properties of thesecompressed compacts such as mechanical strengths, hardness, gas andliquid impermeability can be further improved (at a slight expense ofreduced electrical and thermal conductivity) by bonding the compact asdescribed hereinbefore.

The glass-bonded compressed graphite has a particular advantage in thatit overcomes temperature limitations of the polymer bonded vermiculargraphites. The polymerbonded graphites generally undergo polymerdecomposition at sustained temperatures above about 250 C. The glassbonded composites are useful at temperature above this limit.

However, if the product desired is to be a carboncemented densevermicular graphite compact, the charyielding bonding agent-vermiculargraphite blend should be baked at a temperature within the range of fromabout 800 to about 1200 C. until essentially all the volatileconstituents are evolved. The ordinary range of residue carbon cement insuch a carbon-cemented graphite structure is from about 1 to about 50weight percent and preferably from about 5 to about 30 weight percent ofthe final product. Generally, as the carbon cement fraction decreasesbelow about 1 weight percent, the structure resembles the unbondedcompressed vermicular graphite. If the carbon cement content of thefinal product exceeds about 50 weight percent, the advantageouselectrical and thermal conductivity properties of the expanded graphitefraction may be diminished.

Char yielding bonding agents suitable for use herein include, foreaxmple, asphalt, tar, sugars, phenol formaldehyde resins. If necessaryfor ease of mixing and shaping, a solvent such as xylene, kerosene, andthe like can be used. While compressed expanded graphite in its variousforms possesses an ability to withstand high temperature oxidation whichis superior to almost all other forms of graphite, this characteristicmay be enhanced to a remarkable degree by the addition thereto of minoramounts of oxides of boron or phosphorous or salts of borates andphosphate. Quantities of these additives of from 0.5 weight percent toabout 10 weight percent Will reduce the high temperature oxidationweight loss in the compressed vermicular graphite to as low as -25percent of the value obtained without such additive. This isaccomplished without any detrimental effect upon the other physicalproperties of the compacted graphite. The additive compounds such as B 0Na B O;,

AlPO Zn (PO and the like are added to the low bulk density vermiculargraphite as fine powders (at least mesh and preferably less than 325mesh in particle size). The mixture is then blended by such methods astumbling and then compressed into the desired form.

In addition, bonded compressed vermicular graphite foil laminates can beconstructed in accordance with the instant method. The graphite foilhaving a thickness of from about 0.001 to about 0.10 inch or greater,and preferably from about 0.003 to about 0.015 inch is prepared asdescribed hereinbefore. Layers of the product graphite foil are bondedto one another with a bonding agent in a weight ratio of bonding agentto foil, of from about /2 to about /2. The bonding agent can be a liquidpolymerizable monomer or prepolymer of a solid finely divided powder ora foil or film of the solid polymer, or char yielding substances.Examples of bonding agents that are suitable for use herein includepolyethylene powder, polyethylene film, liquid phenol formaldehyderesins, powdered phenol formaldehyde resins, epoxy resins polystyrenefilm, polymethyl cellulose film, fluorocarbon resins, silicone resins,polyesters, acetal copolymers, polyamides, polycarbonate resins, coaltar pitches, natural asphalts, sugars, saccharides, inorganic glassesthat wet graphite such as B 0 borates, P 0 and V 0 and the like.

The so-produced bonded-expanded vermicular graphite foil laminate hasextremely high electrical and thermal conductivity in the preferreddirections (parallel to the plane of the foil) and unusually highelectrictl and thermal anisotropy, and is highly liquid and gasimpermeable.

Furthermore, expanded vermicular graphite itself or a previously formedfoil of the same can be compressed and bonded to polymeric and othertypes of flexible and non-flexible essentially non-electricallyconductive substrates such as Saran film, polyethylene film, polyvinylchloride film, paper, rubber, polystyrene, polymethyl methacrylate,polycarbonates, glass cloth, asbestos paper, ceramic and glass articlesand the like. Where the polymer substrates do not have useful adhesionproperties the bonding between the graphite surface and thenon-conducting substrate can be accomplished by the use of conventionalorganic or inorganic adhesives which include for example, rubbercements, animal glues, sodium silicate solutions, epoxy resins, phenolformaldehyde resins, and the like. The electrically non-conductingsubstrates impart mechanical strengths to the graphite film and serve asan electrical or thermal insulation medium. Such substrates may totallyenclose the graphite film as well as cover only a single surface.

Also, expanded vermicular graphite itself or a previously formed foil ofthe same can be compressed and bonded to flexible or non-flexibleelectrically conductive substrates. Electrically conductive substratessuch as metal rods, bars, sheets or foils (Al, Mg, Cu, Mo, Fe, Ni, Ag,T1, for instance) are coated with graphite according to this invention.Generally such substrates require a bondmg agent to obtain adherence ofthe graphite such as, water-glass, organic adhesives, protein glues, andother commonly available cementing or bonding agents. This affords ameans for providing electrical or thermal conductors with a protectivebarrier coating which is resistant to corrosive or chemical attack, yetthe coating is itself electrically and thermally conductive.

'An improved form of a flexible, dense, impermeable graphite sheetmaterial having excellent strength characteristics has been obtained byinternally reinforcing the graphite compact with a metal mesh. Such ametal meshgraphite composite is prepared by providing a layer of ployedon a weight proportion basis of from about 0.2- 2/1 (acidmember/graphite). These are maintained in contact for at least about oneminute, although a lengthy contact time of hours or days is notdetrimental. The acid treated graphite, now expandable, is separatedfrom 10 ular sample of graphite had not been acid-treated nor expanded.In addition compacts were prepared from 200 mesh flake graphite (DixonNo. 635).

The several samples obtained were submitted to vari ous physical teststo determine some of their physical any excess and washed and dr1ed, 1fdesired. The acldproperties. Table I summanzes some of these properties.

TABLE l.FLAKE GRAPHITE Property Strength Specific Resistresistance, anceCommicrohm inches anisot- Tenpresropy sile, sive Compact type ratiop.s.i. p.s.i.,

Expanded graphite (Dixon N o. 1) 34, 400 161 214:1 943 8, 900 Unexpandedgraphite (Dixon No. 1) 939 282 3. 33:1 5 500 Fine graphite powder (DixonNo.- 635) 1, 960 460 4. 26:1 37. 5 1, 200

1 Parallel to compression vector;

NOTE:

2 Perpendicular to compression vector.

ified graphite is then heated until exfoliation or expansion occurs. Thepreferred method of heating is to contact the acidified graphite with ahydrocarbon flame (for example, a propane flame).

Alternatively, another method of preparing the expandable graphite whichis subsequently expanded for use in the method of the present inventionis to treat with an aqueous peroxy-halo acid, preferably perchloric orperiodic acid, using an acid concentration of from about 2 to about 70weight percent or more and an acid/ graphite weight proportion of fromabout 0.5-2/ 1. The acid treated graphite, now expandable, is separatedfrom excess acid, and heated to give the expanded feed stock,

The crystalline graphite also can be anodically electrolyzed in anaqueous acidic or aqueous salt electrolyte at an electrolyte temperatureof from about 0 to about 80 C. at a minimum cell potential of about 2volts. The total quantity of electricity passed is equivalent to fromabout to about 500 ampere-hours per pound of graphite. Electricallytreated graphite, now expandable, is separated from the electrolytesolution, and heated sufficiently to cause expansion or exfoliation ofthe graphite flakes. The so-formed expanded graphite feed stock has abulk density as low as 0.1 lb./ft. and preferably less than 2 lb./ft.

The following examples serve to further illustrate the method of theinstant invention but are in no way meant to limit it thereto.

EXAMPLE I The following procedure was carried out in order to comparethe properties of compressed graphite structurcs prepared from expandedgraphite in accordance with the instant invention as opposed to thosegraphite compacts prepared from unexpanded graphite.

Run A A commercially available natural flake graphite (Dixon No. 1)having a flake size range of from about mesh to about 50 mesh was Wettedwith a mixture of concentrated sulfuric acid plus concentrated HNO(weight ratio of H 80 to HNO being 2:1) and then washed free of excessacid. The so-acidified flake was heated with a propane torch to about1000 C. thereby producing a loose particulate worm-like product havingan apparent bulk density of about 0.18 lb./ft.

A portion of the vermicular graphite was compressed to 10,000 p.s.i. toobtain flat slabs measuring 1% inches wide, 4% inches long, and 0.040inch thick. Another sample of the vermicular graphite was uniaxiallycompressed at 10,000 p.s.i. into blocks measuring 2 inches wide, 1.26inches long, and 0.35 inch thick.

Similar sized slabs and blocks were prepared by uniaxially compressingto 10,000 p.s.i. the same type of natural flake graphite (Dixon No. 1)except that this partic- 635= 200 mesh graphite powder.

It is to be noted that the compressed vermicular product prepared inaccordance with the instant method as compared to the compacts made fromunacidified and unexpanded graphite had an increased resistanceanisotropy ratio of about 50-fold, an increased tensile strength ofalmost ZOO-fold, and an increased compressive strength of more than17-fold.

Run B In another run, a batch of commercially available grade ofparticulate Ceylon crystalline lump graphite (Cummings-Moore Grade No.809) was wetted with a 60 percent HClO solution (1:2 acid-to-graphiteratio) and then heated to about 400 C. thereby producing a fluifyprodnot having a bulk density of about 7 lb./ft. This product wasuniaxially compressed to about 9700 p.s.i. to obtain thirli1 )flexiblestrips (1% inches x 4% inches x 0.023 mc Another batch of untreated andunexpanded crystalline lump graphite was uniaxially compressed to about10,000 p.s.i. to obtain similarly sized flexible strips. Variousphysical properties of these originally crystalline lump graphite, aresummarized in Table I I.

TABLE IL-CRYSTALLINE LUMP GRAPHITE Specific resist- Com ance, Tensilepressive DJJCIOhIIl strength, strength inch p.s.i. 2 p.s.i. 2

Expanded graphite compact 383 191 4, 800 Unexpanded graphite compact 71046 760 1 Perpendicular to compression vector. 2 Parallel to compresslonvector.

It is readily seen that expanding, then compressing the crystalline lumpgraphite improved its bulk conductivity and its tensile and compressivestrengths.

Run C EXAMPLE Ia An excellent utility for the compressed expandedgraphite product of the instant invention is to form it into a highstrength, flexible, impermeable gasket material.

A gasket 6 inches in diameter and Ma inch thick was prepared bycompressing 30 grams of expanded graphite (bulk density .2 lb./ ft. in acylindrical mold of 6 inch inside diameter using 800 lbs. /ft.compression force. Holes were cut in the gasket to conform to fourequispaced bolt positions of a flange of a reactor and the gasket wasclamped into place between the flange and the reactor using only thefour bolts. The temperature of the vessel was raised to 1600 F. and areduced pressure of 30" Hg was employed. The vessel was sealed 01f fromthe vacuum pump and the temperature was held constant. These conditionswere maintained for 3 hours and no increase of pressure within thevessel occurred which indicated an air-tight seal was effected by thegasket. The gasket was removed and inspected and found to be undamagedand re-usable.

Similarly, the vessel was pressured to 3000 p.s.i. (hydraulic waterpressure) and the pressure was maintained for 2 days. No leakageoccurred and the gasket was found to be undamaged and reusable.

To illustrate an additional utility, expanded vermicular graphite wascompressed in a mold to form a dense article in the shape of achevron-type valve packing ring. Several of these rings were used in avalve in place of the commonly used packing rings. The graphite ringswere found to function leak-free for a much longer length of time thanthe packing normally used when put into service in a high temperaturemolten caustic line. The rings were compressible and did not break whenthe packing gland was tightened.

EXAMPLE Ib An industrial grade of large natural flake graphite, commonlyused in crucible manufacture, and having a mesh size range of 14 to 30mesh, was treated with a mixture of concentrated H 80 and concentratedHNO (weight ratio of H 50 to HNO of 2: 1) and the graphite was Washedfree of acid. The acid-treated graphite was contacted with the flame ofa propane torch to produce and expanded graphite having an apparent bulkdensity of 0.18 lb./ft. and being vermicular in appearance. Thisexpanded vermicular graphite was compressed uniaxially in a steel moldunder about 11,000 p.s.i. to yield various sized compacts. The followingdata was obtained by testing the compacts:

TABLE IIA Property: Measurement Density 1.89 gms./cc. Specificresistance in the plane at right angles to the compression vector 131microhm inches. Specific resistance in the plane parallel to thecompression vector 32,500 microhm inches. Anisotropic resistance ratio248:1. Tensile strength in the plane at right a n g l e s to thecompression vector 1970 p.s.i. Compressive strength in a directionparallel to the compression vector 7200 p.s.i. Thermal conductivity inthe plane at right angles to the compression vector given ascal./sec./cm. C./cm. 2.4 at 50 C., i :1.7 at 75 C.,

0.75 at 275 C. Permeability to helium at 23 C. 1.5 X-

cmP/sec.

Similarly (varying the compressive force in the range of 5 to 60,000p.s.i.) compacts have been prepared which had densities in the range offrom about 0.03 to 2.1 gms./cc., tensile strengths up to about 3000p.s.i., compressive strengths up to about 9000 p.s.i. and specificresistances down to about 120 microhm inches.

12 EXAMPLE n Another portion of commercially available material flakegraphite was acidified and expanded as described in Example -I. Theexpanded vermicular graphite product had an apparent bulk density ofabout 0.2 lb./ft. (0.0032 gm./cc.).

The vermicular graphite was then compressed at 50 p.s.i. intoself-cohered blocks having an apparent density of about 0.25 gm./cc. Oneblock was then recompressed at a vector at right angles to the initialcompression vector to a pressure of about 16,000 p.s.i. therebyproducing a compact having an apparent density of about 1.8 grn./cc.

This 16,000 p.s.i. block was found to have specific resistances of about1400, 606, and microhms in the second compression vector, firstcompression vector and uncompressed vector respectively and a maximumelectrical anisotropy ratio of about 9.35 to 1.

Another block was uniaxially compressed to about 16,000 p.s.i. andshowed a specific resistance perpendicular to the compression vector ofabout 146 microhms inches and a specific resistance parallel to thecompression vector of about 24,600 microhm inches. Thus, this axiallycompressed block had an electrical anisotropy ratio of about 169 to 1.

It is to be noted that biaxially compressed expanded graphite had amarked decrease in electrical anisotropy as compared to the uniaxiallycompressed expanded graphite.

In a third run, low bulk density expanded vermicular graphite preparedas described hereinbefore, was compressed in a series of molds in threemutually perpendicular vectors in the following sequence:

(A) Compressed to about 30 p.s.i. to yield a compact having a density ofabout 0.196 gm./ cc.

(B) Compressed at right angles to the first compression vector to apressure of about 150 p.s.i. thereby producing a compact having adensity of about 0.694 gm./cc.

(C) Compressed in a vector mutually perpendicular to the first andsecond compression vectors to a pressure of about 10,000 p.s.i. toproduce a triaxially compressed graphite block having a density of about1.72 gms./cc.

The specific resistances were 630, 1260 and 1070 microhms along thefirst, second and third compression vectors, respectively.

Thus. it is seen that the maximum electrical anisotropy ratio of thetriaxially compressed vermicular graphite above was only about 2 to 1whereas with biaxial compression, the electrical anisotropy ratio wasabout 9:1, and an uniaxial compression, the anisotropy ratio can reachvalues as high as 250:1 or more.

EXAMPLE IIa In a manner similar to the foregoing, simultaneouscompression of a pile of vermicular graphite from all directions(isostatic compression) yielded a compact electrically equivalent tosequential triaxial compression.

Vermicular graphite having an apparent bulk density of about 0.2 poundper cubic foot was compressed hydrostatically in a bag to a finalpressure of about 950 p.s.i., to yield a compact having an apparent bulkdensity of 50.6 pounds per cubic foot. Rod segments were cut out of thecompact in three mutually perpendicular axes and measured for specificresistance:

Microhm inches Axis No. 1 1270 Axis No. 2 1760 Axis No. 3 1430 Maximumelectrical anisotropy was found to be 1.39:1 (between Axis No. 1 and No.2) and the minimum anisotropy ratio was 1.12:1 (between Axis No. 1 andNo. 3). Pressure of from about 5 to about 50,000 p.s.i. of hydraulic orpneumatic pressure may be used in this application.

EXAMPLE IIb TABLE III) Compact density Electrical Radial afterresistivity pressure application in axial applied at pressure direction(p.s.i.) (gms./cc.) microhm(in.)

It was found that sheets, foils, or discs could be cut, shaved, orsliced from the resulting essentially cylindrically shaped compact.

EXAMPLE IIc Graphite resistance heating elements were prepared inaccordance with the instant invention as follows:

Vermicular graphite having an apparent bulk density of about 03 poundper cubic foot was fed into a long tube having a diameter of 2.2centimeters. A piston was inserted into the tube which compressed thevermicular graphite to a pressure of about 100 p.s.i. The compressedvermicular graphite rod thereby produced had an apparent density of0.495 gram per cc. and was found to have a specific resistance parallelto the rod axis of 11,800 microhm inches.

The vermicular graphite rod (2.2 cm. dia.) was clamped between two largecopper plates and 600 amps (A.C.) was passed through the rod at apotential of 13 volts across 6 centimeters of rod length. This rodexhibited a surface temperature of 235 C.

Another rod was compressed from vermicular graphite in the same tubemold to a pressure of about 300 p.s.i. This product rod had an apparentdensity of 1.03 gram per cc. and was found to have a specific resistanceof 17,600 microhm inches or was electrically equivalent to a graphiterod 50 times longer at the same diameter.

The second vermicular graphite rod (1.03 grams per cc.) was connectedacross an arc welder in a fashion similar to the above example and 740amps was passed through the rod with a potential drop of 24 volts acrossa 3.8 cm. long rod. The heating rod surface temperature was observed tobe 2540 C. (optical pyrometer) at a power density of 710 watts per cubiccentimeter of heater rod.

Thus it is seen that graphite rods formed by compressing eXpandedvermicular graphite in such a way that the compression vector wasparallel to the current path of the finished heating element, exhibitshigh electrical resistance values along the vector parallel to thecompression vector. The vector perpendicular to said compression force,however, exhibited high thermal conductivity.

EXAMPLE IId Run 1 Highly conductive, highly anisotropic compacts wereprepared by gently blending vermicular graphite having an apparent bulkdensity of about 0.3 pound per cubic foot with varying ratios of copperpowder (-100 mesh). These mixtures were then compressed uniaxially in asteel mold to a pressure of 10,000 p.s.i. to yield cohered compactscontaining 47.5 weight percent copper and 75 weight percent copper.

Another set of compacts was prepared by blending powdered natural flakegraphite (unexpanded) with the same copper powder, and this mixture wasalso compressed uniaxially to 10,000 p.s.i. in the same mold to yieldcompacts containing 50 weight percent and 75 weight percent copper.

Electrical resistance measurements were made on each of the compacts,both in a direction parallel to the initial compression vector andperpendicular to the compression vector. The summarized data is shown inthe following table.

TABLE IId Specific resistance, ohm inch Parallel Perpend. Anisotto comp.to comp. ropy Compact type vector vector ratio Comp. vermicular graphiteplus 47.5

wt. percent Cu 5,300 209 168:1 Comp. vermicular graphite plus 75 wt.percent Cu 960 63.4 15. 2:1 Natural flake graphite plus 50 wt.

percent Cu 1,050 323 3. 25:1 Natural flake graphite plus 75 wt.

percent Cu 25 10. 9 2. 29:1

Run 2 An additional highly conductive highly anisotropic compact wasprepared by blending silver powder with low bulk density vermiculargraphite, then compressing uniaxially to about 10,000 p.s.i. to yield acompact containing 62.1 wt. percent silver. This compact was then heatedto 940 C. to form a sintered mass having an apparent density of 2.82grams per cc. This composite was found to have a specific resistancevalue of 18,900 microhm inches parallel with the compression vector and126 microhm inches perpendicular to the compression vector giving anelectrical anisotropy ratio of approximately :1.

Run 3 Another metal-expanded graphite compact was prepared by gentlyblending 5.5 grams of low bulk density vermicular graphite with 33 gramsof ultrafine nickel powder (derived by decomposing nickel formate). Thismixture was uniaxially compressed in a mold to about 10,000 p.s.i. toyield a cohered composite. Then this composite was heated in an inertatmosphere to 1100 C. to form a sintered compact having a specificresistance in the preferred conductivity direction (perpendicular to thecompression vector) of 187 microhm inches and an electrical anisotropyratio of 12.2:1.

Run 4 Another metal-expanded graphite compact was prepared by blendingsufiicient silicon powder (200 mesh) with vermicular graphite and thenuniaxially compressed to 10,000 p.s.i. to yield a compact containing47.5 weight percent silicon. Specific resistance of this compact in thepreferred direction was 437 microhm inches and parallel to thecompression vector, the value was 34,300 microhm inches, yielding anelectrical anisotropy ratio of 79: 1.

Other graphite composites containing boron, lead, zinc, and iron werealso found to show anisotropic properties.

EXAMPLE IIIa A natural flake graphite having a flake size in the rangeof from about 20 to about 50 mesh was acid-treated and flame expanded ina manner similar to that described in the previous examples. Theapparent bulk density of this vermicular graphite was about 0.2 lb./ft.A plate-type mold was filled with an approximately 3 inches thick layerof the vermicular graphite and uniaxially compressed to about 10,000p.s.i. thereby producing a highly flexible graphite foil. The foil had athickness of about 0.006 inch, a density of about 1.7 gms./cc., and aspecific resistance in the plane .of the foil of about microhm inches.

In a manner similar to the foregoing, sheets having a thickness of about0.001 or 0.002 inch can easily be produced by reducing the thickness ofthe original pile height of expanded graphite and compressing or rollingthe preformed sheet.

In addition, compressed vermicular graphite coatings were applied tovarious substrates by compressing, rubbing, bonding, rolling and thelike to various surfaces such as Saran film, polyethylene film,polyvinyl chloride tapes, paper masking tapes, glass cloth tapes,asbestos paper, ceramic plates, Pyrex plate, and the like, to impartmore electrically conductivity to the surface at a given loading (i.e.grams of graphite per unit of surface area) than the previously knownvarieties of particulate graphites or carbon.

EXAMPLE IIIb A thin coating of graphite on a metallic substrate wasprepared by the following procedure: a 6-mil thick foil, prepared bycompressing expanded vermicular graphite, was bonded to a 10-mil thicksheet of a magnesium base alloy by using a thin coating of rubberadhesive between the metal and graphite. This yielded a highly flexiblegraphite-coated metallic substrate which was electrically conductiveacross the thickness of the sheet. The liquid impermeable graphitecoating was found to be an effective barrier coating againstconcentrated HCl and other corrosive agents.

Similarly, other substrates such as iron, aluminum, steel, nickel, andcopper articles have been coated with graphite using bonding agents suchas epoxides, molten organic polymers, silicone adhesives and animalglues. Also metallic articles have been coated with graphite by firstcoating the article with a tacky or pressure-sensitive adhesive, thenrubbing or pressing expanded vermicular graphite onto the adhesive. Theso coated graphite substrates were found to electrically conduct acrossthe thickness of the substrate. Further the liquid impermeable coatingwas an effective barrier against corrosive agents.

Other conductive substrates such as aluminum and copper foils may alsobe used. The graphite coating gives the metal foil a protective coatingwithout interfering substantially with its electrical properties.Adherence of graphite to the metal substrate is improved by using a thinfilm of adhesive but this is not required for a successful coating.

EXALMPLE IIIc Run 1 Metal mesh-reinforced graphite sheets were preparedin accordance with the instant invention as follows: Expanded vermiculargraphite, having an apparent bulk density of about 0.4 pound per cubicfoot, was poured onto a mold to form a layer about 3 inches deep. Asheet of 100-mesh copper screen was placed on the pile of vermiculargraphite and then another 3-inch deep pile of more vermicular graphitewas placed on the copper screen. The layered system was then compressedto a pressure of about 16,000 p.s.i. to yield a well cohered, flexible,impermeable metal mesh-reinforced graphite sheet. This sheet, containing41.8 weight percent graphite, having a thickness of .023 inch, had asurface resistance of 1.58 10- ohms per square and a specific resistanceof 36.2 microhm inches.

Run 2 The above procedure was repeated using a 200-mesh nickel screen asthe reinforcing mesh. The sheet, having a thickness of .016 inch, andcontaining 61.4 weight percent graphite, was found to have a specificresistance (in the plane of the sheet) of 62.9 microhm inches.

Run 3 The above procedure was repeated by using a heavy gauge l6-meshnichrome screen as the reinforcing agent. The compressed, reinforcedsheet, containing 25.3 weight percent graphite and having a thickness of.035 inch, was found to have a specific resistance of 240 microhminches. The tensile strength of this reinforced sheet was in excess of8,000 p.s.i.

Run 4 The above procedure was repeated by using an 18- mesh galvanizediron screen as the reinforcing agent. The compressed (10,000 p.s.i.),reinforced sheet containing 60.8 weight percent graphite, having athickness of .035 inch, showed a specific resistance (in the Plane ofthe sheet) value of 292 microhm inches.

Run 5 An 18-mesh bronze screen reinforced flexible graphite sheet,containing 36.9 weight percent graphite, was prepared by compressing thelayered graphite-screen mass to about 700 p.s.i.; then the low densitypreformed sheet was further compressed by passing through a set ofrolls. The .0l8-inch thick sheet, having a weight of about grams persquare foot, showed a specific resistance in the plane of the sheet of66 microhm inches. The tensile strength of the bronze screen-reinforcedfoil was found to be 6600 p.s.i. The helium permeability was 9.9x 10-square centimeters per second (across the sheet thickness).

EXAMPLE IV The following procedure was carried out in order to show acomparison of properties of polymer-bonded compressed vermiculargraphite made in accordance with the instant invention as opposed topolymer-bonded graphite flake, graphite powder and graphite cloth.

Run A A series of compacts were prepared as follows: Microfinepolyethylene powder, having an average particle size of about -325 mesh,was dry-blended at weight percents of 5, 15 and 25 with the followingforms of particulate graphite:

(a) Highly expanded vermicular graphite (Dixon No. 1) having an apparentbulk density of about 0.2 lb./ft. and prepared as described in previousexamples.

(b) Unexpanded flake graphite (Dixon No. 1).

(c) A finely powdered natural flake graphite (Dixon No. 635) passingthrough a 200 mesh screen.

The various polymer-graphite blends were poured into a mold anduniaxially compressed to a pressure of about 10,000 p.s.i. to produceflat slabs. These slabs were heated to about 400 'F. for about 30minutes to fuse the polyethylene binder.

Additional compacts of graphite were prepared containing no polymerbinder.

Each of the compositions was prepared in triplicate and the followingproperties were measured:

(1) Specific resistance in the plane perpendicular to the compressionvector.

(2) Tensile strength perpendicular to the compression vector.

Table IVa lists the average results of these property measurements.

TABLE IVa Specific Weightresistauce Tensile percent (microhm strengthGraphite type polymer in.) (p.s.i.)

vermicular graphit 0 1, 600 5 231 1, 25a 15 296 1, 590 25 357 1,596 504, 950 0 280 4 5 4, 400 385 15 40, 700 583 25 92, 000 877 0 460 38 5 1,050 355 15 1, 450 1,370 25 14, 500 1, J03

Determination of electrical anisotropy ratios of the polymer-bondedvermicular graphite showed them to range from about 150:1 to about250:1. By contrast, the electrical anisotropy ratio of the graphiteflake and powder composites showed them to range from about 2:1 to about:1.

Run B TABLE IVb Specific re- Tensile sistance in strength high cond.parallel to Applied pressure, 2nd vector Density direction (mispec. res.(p.s.i.) (g./cc crohm in.) (p.s.i.)

Note that the density and the specific resistance is sensitive to theapplied forming pressures whereas the tensile strengths are less so.

Run C Another set of vermicular graphite compacts was prepared from purevermicular graphite and weight percent polyethylene compressed to adensity of 0.17 gram per cc., then recompressing these at right anglesto the first compression vector to a final pressure of 12,500 p.s.i. Thbiaxially compressed composite containing polyethylene was fused asdescribed in Run B. The properties measured on these composites arelisted below.

Run D Expanded graphite having an apparent bulk density of 0.4 pound percubic foot was blended, by tumbling, with 15 weight percent polyethylenepowder (-325 mesh). This mixture was compressed into a block having anapparent density of 0.21 gram per cc., then compressed, at right anglesto the first compression vector, to a block having a density of 0.775gram per cc. Finally, the block was compressed in a vector normal to thefirst and second compression vectors to a final density of 1.55 gramsper cc. (requiring a pressure of about 14,000 p.s.i.). The block washeated to fusion temperature of the polyethylene.

The resulting composite was found to have the following specificresistances:

Parallel to the 1st compression vector- 10l0 microhm inches Parallel tothe 2nd compression vector-+2070 microhm inches Parallel to the 3rdcompression vector 816 microhm inches 18 Run 13 In a manner similar tothe foregoing other polymer binders such as vinyl chloride resins,polytetrafluoroethylene resins, solid epoxide powder resins and phenolformaldehyde resins when used as a binder for a vermicular graphite(having an approximate bulk density of about 0.2 lb./ft.) producedcomposites which were as conductive or more conductive than commerciallyavailable pure synthetic polycrystalline graphite as shown in Table IVe.In addition, biaxially and triaxially compressing the variouspolymer-bonded vermicular graphites resulted in a marked decrease inanisotropic properties.

TABLE IV e Specific Weight resistance percent (microhm Polymer binderbinder inches) Vinyl chloride resins 5 181 Do 10 209 Do 20 330Polytetrafluoroethylene resins 5 184 Do 15 210 D0 25 v 259 Do 35 365Solid epoxide powder resins 5 176 Do 15 226 D0 25 333 Do 35 440 Phenolformaldehyde reslns 5 204 Do 15 268 D0 25 344 Polyvinylidene fluorid 15200 Polychloroether- 15 r 204 Polycarbonate 15 208 Nylon 15 196 Run FGraphite foil was prepared essentially as described in Example III withthe exception that the expanded graphite was uniaxially compressed toabout 700 p.s.i. thereby reducing the vermicular graphite from a loosepile about 3 inches thick to compacted sheets about .025 inch thick.These preformed sheets were passed through a successive set of rollersuntil their thickness was about 0.010 inch. This foil was found to havea density of 1.77 gms./cc., exhibited tensile strength in the plane ofthe foil of 2080 p.s.i., a specific resistance in the plane of the foilof 144 microhm inches, and a helium permeability value of 1.4 10- cm./sec.

Run G A laminate was formed by alternately laying graphite sheets(formed as in Example 111, Run B) and a 0.010 inch thick polyethylenefilm (approximately 49 layers of each). The composite laminate wascompressed to about 1000 p.s.i. and heated to about C. for about 5minutes to fuse the laminate. The resulting laminate contained about28.5 weight percent polyethylene, had an apparent bulk density of about1.1 gm./cc.

For comparison purposes a graphite cloth-polymer bonded laminate wasprepared as follows. A commercially available graphite cloth (NationalCarbon, WCB grade) was cut into squares and alternately laminated withpolyethylene sheets. The stacked laminate was compressed to about 1000p.s.i. and heated to about 125 C. for about 5 minutes to fuse thestructure. The resulting graphite cloth laminate contained about 29weight percent polyethylene and had an apparent density of about 0.69gm./ cc.

Various physical properties of these 2 types of graphitecontaininglaminates are listed in Table IVg.

TAB LE IVg Graphite Graphite ioil cloth laminate laminate Density,g./cc 1. 1 0.69 Specific resistance along the laminations, ohm

inches 345 X10 2, 800X10-6 Specific resistance across the laminations,ohm

inches I 328 I 3. 12

In addition to the large electrical anisotropy ratio, the vermiculargraphite laminate made in accordance with the instantmethod was found tobe a highly impermeable structure capable of flexure over long lengths.The graphite cloth laminate was both highly gas and liquid permeable.

In a manner similar to that described in Run B of EX- ample III, sheetsof methyl cellulose film in one case and polystyrene film in anothercase were stacked alternately with compressed vermicular graphite sheetsand cured to produce laminates. These laminates had excellent resistanceanisotropic characteristics (40,700:l and 7,200,000:l for methylcellulose and polystyrene, respectively).

Similarly, phenol-formaldehyde powdered resins and epoxy resins wereused as binders for laminated vermicular graphite structures resultingin laminates having exgellent resistance anisotropic properties.

It was further found that if perforations were made in the graphite foillayers then contact could be made through the perforations to obtainpolymer layer to-polymer layer bonding sites. Thus the tensile strengthof the composite in the direction perpendicular to the laminations wasimproved.

' In another modification, a graphite foil (.013 inch thick), madeaccording to this invention, was coated with a liquidphenol-formaldehyde resin and then rolled on a Vs-inch diameter woodenmandrel. The foil was cured thereby yielding a graphite laminate pipehaving an I.D. of %-inch and a Az-inch wall thickness; its specificresistance along the axis of the pipe was found to be about 372 microhminches.

Also, laminations of alternating layers of graphite foil and metal foils(or sheets) were prepared using a bonding agent between the graphite andthe metal.

EXAMPLE V Carbon bonded compressed vermicular graphite was prepared inthe following manner.

Vermicular graphite, having an apparent bulk density of about 0.31b./ft. and prepared by a previously described method from acommercially available material flake graphite (having a particle sizeof from about to about 70 mesh), was gently blended by tumbling with 20weight percent coal tar pitch powder (having an average particle size ofless than 200 mesh). This mixture Was then compressed in a mold,uniaxially, at a pressure of about 10,000 p.s.i. The resulting compact,retained in a steel form, was slowly pyrolyzed in an essentiallyoxygenfree atmosphere over an 8 day period at a maximum temperature of950 C.

The carbon-bonded vermicular graphite product was found to have anapparent density of about 1.65 gms./ cc., a specific resistance of about177 microhm inches in the plane perpendicular to the compression vectorand about 7560 microhm inches in the direction parallel to thecompression vector. This compact thus exhibited an electrical anisotropyratio of about 42.7 to 1.

Another compact was prepared by blending natural flake graphite, thesame as used in the above example in the unexpanded state, with 20weight percent pitch powder, compressed in a mold to 10,000 p.s.i.,uniaxially, and pyrolyzed in a manner identical to the previouslymentioned composition.

This carbon-bonded flake graphite compact was found to have an apparentbulk density of 1.75 grams per cc. and a specific resistance valueperpendicular to the compression vector of 221 microhm inches. Thespecific resistance of this compact parallel to the compression vectorwas 243 microhm inches which results in an electrical anisotropy ratioof about 1.1 to l.

Notethat the carbon-bonded vermicular graphite compact shows a slightlyhigher conductivity in the preferred direction than the carbon-bondedflake graphite compact, but the more striking difference is in the factthat the vermicular graphite is highly anisotropic whereas the flakegraphite compact is essentially isotropic. These two composites alsoshowed a striking difference in the compressive strength parallel to thecompression vector: Carbonbonded flake graphitefailed at 1520 p.s.i.;Carbonbonded vermicular graphitefailed at 7200 p.s.i.; and the flakegraphite compact was highly gas permeable whereas the vermiculargraphite was relatively impermeable.

Another portion of the highly expanded vermicular graphite was blendedwith 40 weight percent pitch powder, then compressed in a mold to anapproximate apparent bulk density of 0.2 gram per cc. The preformedcompact was then rotated degrees and recompressed to about 10,000 p.s.i.to yield a biaxially compressed slab. This slab was then pyrolzed in amanner similar to the previous examples to yield a carbon-bondedbiaxially compressed vermicular graphite slab which possessed thefollowing properties:

(1) Density=about 1.31 grams per cc.

(2) Specific resistance in the uncompressed direction:

about 231 microhm inches (3) Specific resistance parallel to finalcompression vector=about 948 microhm inches (4) Electrical anisotropyratio- 4.1: l

(5) Tensile strength (parallel to uncompressed vector)=about 1620 p.s.i.

(6) Helium permeability=about 1.2 10- cmF/second Another portion of the0.3 gm./ cc. vermicular graphite was compressed to a pressure in excessof about 2000 p.s.i. thereby producing a flexible graphite foil having athickness of about 0.010 inch.

This foil was cut into sheets and then each sheet was liberallysprinkled with a fine coal tar pitch powder (325 mesh) and stacked inlayers. The resulting laminated layer was then compressed to aboutp.s.i. and heated to 400 C. under pressure. The elevated temperaturecaused the excess pitch to be forced out of the laminate. The laminate,upon cooling, was found to contain 5 weight percent coal tar pitch as abinding agent. Next, the laminate was clamped in a steel mold (toprevent distortion during the rapid bake-out), placed in an inertatmospheric oven and heated according to the following schedule:

(a) Gradually increased the temperature to 900 C. in a 6-hour period.

(b) Maintained the temperature at 900 C. for 1 hour.

(0) Gradually cooled.

The resulting graphite foil laminate was found to have a density of 1.24grams per cc. This laminate was found to possess the followingproperties:

(a) Specific resistance parallel to the graphite laminations of about186 microhm inches.

(b) Specific resistance perpendicular to the graphite laminations ofabout 18,000 microhm inches.

(c) Electrical anisotropy ratio of about 96.821.

(d) Compressive strength (compression force perpendicular to thegraphite foil laminations) of about 8960 p.s.1.

(e) Transverse breaking strength.

Force applied perpendicular to laminations-about 2020 p.s.1.

PIorce applied parallel to the laminations-about 3580 p.s.1.

For comparison, a carbon-bonded graphite cloth laminate was prepared inthe following manner: A commercially available graphite cloth (NationalCarbon Co.) was cut into strips and each strip was liberally wetted withmolten coal tar pitch. Then each strip was stacked, one on the other(while the pitch was molten), to form a laminated structure. Thisstructure consisted of 31.4 weight percent graphite cloth. Thepitch-bonded laminate was bolted between two strips of a mild steelholder (to maintain the shape of the laminate during pyrolysis) andpyrolyzed by the following schedule:

(a) Slowly heated to 500 C. over a 20-hour interval,

(b) Then increased the temperature to 980 C. over a 4-hour interval,

(c) Maintained the pyrolyzed composite at 980 C. for 2 hours,

(d) Then cooled.

The resulting carbon bonded graphite cloth laminate was found to have anapparent density of 0.85 gram per cc. and was found to be highly gaspermeable.

The specific resistance of this laminate parallel to the graphitelaminations was found to be 5,900 microhm inches (over 30 times greaterthan the carbon-bonded graphite foil laminate). The specific resistanceperpendicular to the laminations was found to be 18,000 microhm inches.

The electrical anisotropy ratio of this composite was 3.08:1 whereas thecarbon-bonded graphite foil laminate was 96.8:1. Compressive strength ofthis laminate in its strongest direction was 2260 p.s.i.

Graphite foil, prepared in the same manner as described above, having athickness of 0.010 inch, was coated with a molten petroleum pitch andthereafter was wrapped around a l-inch diameter graphite rod mandrel.After wrapping on about a dozen layers of graphite foil, the laminateand mandrel were heated to about 200 C. and the mandrel was removed fromthe laminate. Next the pitch-bonded laminate was heated in inert gasenvironment to about 500 C. for 8 hours, and then the temperature wasincreased to 950 C. in 4 hours.

The pyrolyzed carbon-bonded graphite foil laminated pipe was 1% inch OD.and had an average thickness of about A: inch. This pipe was found tohave a helium permeability of 4.0 10- cc. per second on one cubiccentimeter at a pressure differential across the walls of oneatmosphere. This value is in the order of 1000 times less permeable thancommercially available polycrystalline Acheson-Process syntheticgraphite.

EXAMPLE VI In another embodiment of the instant invention, a compacthaving a low density and a relatively high binder content was preparedby admixing expanded graphite of a bulk density of about 0.2 lb./ft.with about 35 weight percent of a phenol-formaldehyde binder. Themixture was compressed uniaxially under 75 p.s.i. compression force intoa compact. The compact was then pyrolyzed at 1000 C. After pyrolysis ithad a density of about 0.31 gm./cc.

Similarly, a mixture containing expanded graphic and weight percentfinely powdered polyethylene was uniaxially compressed under about 25p.s.i. compression force into a compact. The so-formed compact washeated to about 180 C. to fuse the polyethylene. The resulting structurehad a density of 0.175 gm./'cc. The specific electrical resistance wasmeasured parallel to the compression vector and found to be about 2760microhm inches. The specific resistance perpendicular to the compressionvector was about 1630 microhm inches. Thus the anisotropic ratio wasabout 1.7 to about 1.0.

EXAMPLE VII Glass-bonded 'vermicular graphite composites were preparedas follows: About 22 grams of a highly expanded vermicular graphite(apparent bulk density of about 0.2 lb./ft. prepared as describedhereinbefore; were blended, by gentle tumbling, with about 22 grams of acommercially available lime glass powder having an average particle sizeof about 325 mesh. This mixture was uniaxially compressed to a pressureof about 10,000 p.s.i. thereby producing a rectangular slab. Theresulting compact was bolted into a steel form '(to prevent distortionduring the fusion operation) and then heated in an inert atmospherefurnace to maximum temperature of about 940 C.

The resulting glass-bonded vermicular graphite possessed the followingproperties:

(1) Density about 2.25 gm./cc.

22 (2) Specific resistance:

(a) perpendicular to the compression vector--about 460 microhm inches(b) parallel to the compression vector-about 30,200 microhm inches (3)Electrical anisotropyabout 65.6:1.

In contrast, a composite prepared by blending a 1:1 weight ratio ofnatural flake graphite (the same variety used to prepare the vermiculargraphite) and lime glass powder, then compressing the mixture to about10,000 p.s.i. and fusing the glass by heating to 950 C. exhibited thefollowing properties:

(1) Specific resistance:

(a) perpendicular to the compression vector-about 14,800 microhm inches(b) parallel to the compression vector-about 32,800 microhm inches (2)Electrical anisotropy ratio2.22: 1.

Thus, it is readily seen that the glass-bonded vermicular graphite isabout 30 times as conductive in the low resistance direction compared tothe glass-bonded flake graphite and exhibits a substantially largeanisotropic ratio.

In a manner similar to the foregoing, boron oxide powder, and mixturesof lime glass and boron oxide or sodium tetraborate were blended withvermicular graphite to produce glass bonded graphite which hadproperties similar to those described for the lime glass-graphitecompact.

EXAMPLE VIII About twenty-two grams of vermicular expanded graphitehaving an apparent bulk density of about 0.3 lb./ft. were gently blendedby tumbling with about 2 grams of finely powdered Ca (PO (i-324 mesh).The blended mixture was then compressed uniaxially in a rectangularsteel mold to about 10,000 p.s.i. to yield a well cohered, electricallyand thermally anisotropic compact containing 8.34 weight percent calciumphosphate.

Another composite was prepared in the following way: about 23 grams ofvermicular graphite was blended with 2 grams of finely divided anhydrousB 0 This mixture was also compressed uniaxially to 10,000 p.s.i. toyield a well cohered, anisotropic graphite compact containing about 8weight percent B 0 Still another compact was prepared from the samevermicular graphite, containing no additives, by compressing thegraphite in the same mold to 10,000 p.s.i.

These three samples, along with an identical-sized piece of commercialpolycrystalline graphite (National Carbon Company AGSR) were each placedin a separate 3-inch diameter cylindrical Vycor reactor. These reactorstfitted with an inlet and exit tube for air passage, were each placed ina furnace, heated to 500 degrees C., and an air flow of cc. per minute(S.T.P.) passed over the contained heated graphite samples. After 60hours of sustained air oxidation of the graphite samples at 500 degreesC., the samples were cooled and weighed for oxidation loss. The resultsare summarized in the table below.

Graphite oxidation rate at 500 C. (gms. loss/ Graphite type: hr./ft.surface) (1) Commercial polycrystalline (National Carbon AGSR) 0.904

(2) Pure biaxially compressed vermicular graphite 0.332

(3) Biaxially comp. verm. graphite containing 8.34% Ca -(PO' 0.080

(4) Biaxially comp. verm. graphite containing B203 23 IEXAMPLE IX Apiece of carpet of pile construction, 2 feet x 8 feet in size, wascoated on its underside by a latex. While the latex was still wet,expanded graphite, prepared as described hereinbefore, having a bulkdensity of about 0.2 lb./ft. was compressed at a pressure of about 10 to15 p.s.i. onto the latex to obtain a covering layer of graphite of about0.01 inch in thickness. A strip of copper screen wire, about 3 inches by22 inches in size, was embedded in each of the two ends of the graphitelayer. Two conductor wires were silver-soldered to each piece of screenand were also attached to a two-prong male plug-in device. Another coatof latex was applied to the graphite layer so as to insulate thegraphite. It was found that the latex penetrated the graphite layer atrandom points and made contact with the previous latex coating so thatthe graphite was held firmly in position.

The rug was plugged into a power source comprising a Variac and voltagesranging up to about 115 volts were applied. The rug heated uniformly, upto about 120 F. on the upper surface. The rug did not become hot enoughto overheat the rug or its components nor injure the vinyl tile floor onwhich it was laying.

In another run, a strip of flexible urethane foam (2 feet x 8 feet xinch in dimension) was glued to the underside of the carpet. It wasfound that the use of the foam retarded loss of heat into the floor, butdid not cause overheating of the rug.

Various modifications can be made in the instant invention withoutdeparting from the spirit or scope thereof, for it is to be understoodthat we limit ourselves only as defined in the appended claims.

I claim:

1. A process for the preparation of anisotropic graphite-powdercomposites which comprises admixing vermicular expanded graphite havinga density of from about 0.1 to about 10 lbs/ft. with from 5 to weightpercent, based on the total weight of the composite, of a powderselected from the group consisting of silver, copper, nickel, zinc,lead, silicon, iron and boron and having a particle size of less thanmesh, and compressing nonisostatically such admixture at a pressure offrom about 5 to about 50,000 pounds per square inch in predetermineddirections in a forming apparatus, to a cohered composite having adensity of from 10 to pounds per cubic foot.

2. The process according to claim 1 wherein the powder is silver.

3. The process according to claim 1 wherein the powder is copper.

4. The process according to claim 1 wherein the powder is silicon. v p

5. The process according to claim 1 wherein the powder is nickel.

References Cited UNITED STATES PATENTS 1,137,373 4/1915 Aylsworth23209.1 X 3,003,975 10/1961 Louis 252503 3,114,197 12/1963 DuBois et al29182.5 X 3,142,894 8/1964 ROSS 29182.5 3,191,278 6/1965 Kendall et al29182.5 3,323,869 6/1967 OlstOWSki 23209.1 3,396,054 8/1968 Gremion75201 CARL D. QUARFORTH, Primary Examiner R. E. SCI-IAFER, AssistantExaminer US. Cl. X.R.

KIN-WED Lemme 5 mm? @FMEE I M l @ERTHFHQATEE @l CQRQEQN Patent No. 3,666,h55 name 30 May 1972 Inventor) Franciszek Olstowski It is certifiefithat error appears in the aboveidentifie& patent and that said LettersPatent are hereby corrected as shown helow:

Column 12, line 25, delete "axially" and insert ''-uniaxially-.

Column 18, line 8, change "0.2 lb./ft.)" to --0.2 lb./ft.

Signed and sealed this 10th day of October" 1972.,

(SEAL) Attest:

EDWARD MQFLETOHER JRQ ROBERT GOTTSCHALK Attesting Officer" Goxmnissionerof Patents Po-1oso g;

Patent No. 3,666,h55 Dated 30 May 1972 Inventor) Franciszek Olstowski Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 12, line 25, delete "axially" and insert --'-unieucially.

Column 18, line 8, change "0.2 lb./ft.)" to --0.2 lb./ft.

Signed and sealed thie 10th day of October 1972.

(SEAL) Attesti EDWARD M.,FLETCHER JRQ ROBERT GO'ITSCHALK AttestingOfficer Gorrnnissioner of Patents

